Ethers have become an important gasoline blending component in order to increase the octane rating of the gasoline without exceeding regulatory Reid vapor pressure limits and to reduce carbon monoxide emissions. Processes involving catalytic distillation have been used to produce ethers by the reaction of an alcohol with an isoalkene, see U.S. Pat. No. 5,258,560; in particular, catalytic distillation has been used to produce ethyl tertiary butyl ether by the reaction of ethanol and isobutylene. For example, U.S. Pat. No. 5,248,836 discloses passing an isobutylene-containing stream and a stream containing ethanol and ethyl tertiary butyl ether through a straight pass reactor to selectively react ethanol and a portion of the isobutylene to form a first product stream that is sent to a catalytic distillation column. In the catalytic distillation zone the ethyl tertiary butyl ether is largely distilled from the ethanol and isobutylene that are then further reacted to form a second product stream. The distilled ethyl tertiary butyl ether is collected and the second product stream is recycled to the straight pass reactor. U.S. Pat. No. 5,368,691 discloses a configuration for catalytic distillation whereby reaction zones are alternated with, and clearly separated from, distillation zones without a continuous liquid mass between a reaction zone and an adjacent distillation zone.
However, because excess ethanol is usually used to drive the etherification of isobutylene to high conversion and the unreacted ethanol distills with the product ethyl tertiary butyl ether, it is difficult to obtain an ethyl tertiary butyl ether catalytic distillation product having low levels of ethanol. Removing the ethanol from the ethyl tertiary butyl ether product results in a product having a lower Reid vapor pressure, and therefore more valuable. U.S. Pat. No. 5,158,652 discloses a process for separating ethyl tertiary butyl ether and ethanol using two distillation columns operating at different temperatures and pressures. U.S. Pat. No. 5,401,887 discloses a process where the reactants undergo etherification in a reactor and the reactor effluent is sent to a distillation column for separation. The distillation column bottoms containing both ethanol and ethyl tertiary butyl ether is further processed to separate the ethanol from the ethyl tertiary butyl ether by adsorbing the ethanol on a selective adsorbent. U.S. Pat. No. 4,198,530 discloses a process for producing methanol-free methyl tertiary butyl ether. The methyl tertiary butyl ether is formed in an etherification reactor using a feed that contains methanol, C.sub.4 hydrocarbons, and a significant amount of normal butene. The reactor effluent is passed to a distillation zone where substantially all of the methanol in the effluent is azeotropically removed together with the C.sub.4 hydrocarbons and whereby substantially all of the methanol is removed from the methyl tertiary butyl ether. U.S. Pat. No. 4,413,150 discloses converting isobutylene and an alcohol to an ether and then separating the product ether from the reaction mixture in a single distillation column where the product ether is substantially free of C.sub.4 hydrocarbons and alcohol.
Applicants, however, are the first to realize that by routing a stream of largely normal butane from a C.sub.4 distillation column to an etherification zone at a point prior to the catalytic distillation column, excess ethanol will be drawn into an azeotrope with the normal butane within the catalytic distillation column and will not be able to contaminate the ethyl tertiary butyl ether product. The result is a high purity ethyl tertiary butyl ether product containing no more than 0.6 weight percent ethanol, and preferably 0.07 weight percent ethanol, available directly from the catalytic distillation column. No further downstream processing is needed for removal of ethanol and purification of the ethyl tertiary butyl ether.